1. Field of the Invention
The invention relates to a photosensor, particularly to a photosensor that is provided in a display device to measure the intensity of ambient light.
2. Description of the Related Art
It has been suggested that an ambient light sensor is provided in a display device to measure the intensity of ambient light and correspondingly adjust the light intensity of a light source built in the display device. Thereby, optimum display contrast can be achieved and power consumption is allowed to be reduced.
FIG. 1 shows an equivalent circuit diagram of a conventional photosensor, and FIG. 2 shows an exemplary timing chart of input signals for the photosensor 100 shown in FIG. 1. Referring to both FIG. 1 and FIG. 2, the photosensor 100 includes a sensor transistor Q1, a selection transistor Q2, a current-generating transistor Q3, an output transistor Q4, and a storage capacitor C1. The photosensor outputs a sensor current Iout whose magnitude depends on the amount of received ambient light. The sensor transistor Q1 is supplied with a first voltage VDD and a second voltage VGG. When the selection signal SELECT is in a high level, the selection transistor Q2 is turned on to electrically connect the sensor transistor Q1 and the storage capacitor C1 with the first voltage VDD. At this time, the sensor transistor Q1 does not generate any photocurrent and the storage capacitor C1 is initiated to be charged with the first voltage VDD. Then, when the read signal READ is in a high level, the output transistor Q4 is turned on and outputs the first voltage VDD in response to the read signal READ. On the other hand, when the selection signal SELECT is in a low level, the selection transistor Q2 is turned off to disconnect the sensor transistor Q1 and the storage capacitor C1 from the first voltage VDD. Accordingly, the storage capacitor C1 begins storing electrical charges to generate the photovoltage that is applied to the current-generating transistor Q3. Hence, the magnitude of the sensor current Iout depends on the difference between the photovoltage and the first voltage VDD. Further, when the read signal READ is in a high level, the output transistor Q4 is turned on and outputs the photovoltage whose magnitude is in proportion to the sensor current Iout.
However, according to the above design, the current-generating transistor Q3 is subjected to a long-term negative bias to cause a shift in the threshold voltage of the transistor Q3 to damage the transistor Q3. Besides, since the voltage at node n1 is set as the first voltage VDD during each reset operation, the difference between the photovoltage and the first voltage VDD (serving as a reference voltage) is quite small.
FIG. 3 shows an equivalent circuit diagram of another conventional photosensor 200. Referring to FIG. 3, the photosensor 200 includes a sensor circuit 202, a reference voltage generating circuit 204 and a processor 206. The sensor circuit 202 includes a sensor transistor Q1, a reset transistor Q2, a switching transistor Q3 and two capacitors C1 and C2. The reference voltage generating circuit 204 includes a sensor transistor Q4, a reset transistor Q5, a switching transistor Q6 and two capacitors C3 and C4. The sensor transistor Q1 is supplied with a first voltage VDD and a second voltage VGG, and the two capacitors C1 and C2 are connected to a third voltage VDC. The photosensor 200 is enabled by a gate driver (not shown). Specifically, when the reset transistor Q2 is turned on to perform a reset operation, the switching transistor Q3 is turned on by the output of a first stage of the gate driver to obtain a reference voltage Δ V1 for the switching transistor Q3. Then, after the sensor transistor Q1 receives ambient light for some time, the switching transistor Q3 is turned on by the output of a last stage of the gate driver to obtain a photovoltage Δ V2 for the switching transistor Q3. According to the above design, the reset operation allows for a competently large difference between the photovoltage and the reference voltage. However, such design requires two distinct circuits, the sensor circuit 202 for generating the photovoltage and the reference voltage generating circuit 204 for generating the reference voltage, to cause a considerable number of constituting components and high fabrication costs.